Your eye is an amazing device.
My choice for an observing light is the Wayllshine Red LED Flashlight. Although the light in this image may appear yellow, in reality it is intensely red.
credit: Michael E. Bakich
Today I want to talk about something beginning observers tend to overlook. It a process of the human eye called dark adaptation. In terrestrial terms, it helps us see better at night and in dark places. That makes it important for our safety. For amateur astronomers, however, it’s important because it allows us to see fainter celestial objects.
During dark adaptation, the eyes increase their sensitivity to low levels of light. Rhodopsin (sometimes called visual purple) is the substance in the rods of the eye responsible for this. Our dark adaptation increases as the amount of visual purple in the rods increases. It’s a biochemical reaction I don’t understand. Each person adapts to darkness to different degrees and at different rates. In a darkened theater, the eye adapts quickly. That’s a lot of light, however, compared to a moonless night.
The key time for letting your eyes adapt is the first 30 minutes. During this half hour, the eye’s sensitivity increases 10,000 times. After that, there’s little gain. But if at any time our eyes are exposed to a bright light, nighttime sensitivity drops. How much depends on the intensity and duration of the exposure. Surprisingly, brief flashes from high-intensity strobe lights have little effect on night vision. This is because the pulses are short (milliseconds). Durations of bright light lasting one second or longer, however, can seriously impair night vision. So try to avoid them.
Observing lights
Rod cells are much more sensitive to blue light and are least affected by the wavelength of a dim red light. Red lights do not significantly impair night vision if proper techniques are used. To minimize the effect of red light on night vision, the intensity should be adjusted to the lowest usable level and the illuminated object should only be viewed for a short time.
Please, for your own safety around experienced observers, don’t take a flashlight and wrap a piece of red cellophane around the lighted end. A bright light, even if it’s red, will draw the ire of others. Instead, buy a dimmable red flashlight. As your progress as an observer, you’ll find that the amount of light you use on star charts or in your observatory diminishes greatly. You realize that brighter light means you see less at the telescope, and that your eyes take longer to re-adapt to the darkness.
For those observers advanced in years (and I place myself in this category), the small print on star charts can be difficult to read under low illumination. Inexperienced observers search out brighter lights. Experienced observers use a magnifier. This lets you use a reading light at or near the lowest brightness level. I suggest using a non-illuminated magnifier. I purchased an illuminated magnifier once. The light in it wasn’t very red, not at all adjustable, and far too bright. Good idea. Bad execution.
